There are several types of inductive coin discrimination apparatus based upon passing a coin through the electromagnetic field of an inductor which is part of an oscillator circuit. For greater accuracy of discrimination, a coin may be examined using two or more frequencies by introducing the coin into electromagnetic fields of different frequencies and determining if the interactions betwen the coin being tested and the fields are within predetermined tolerances anticipated for acceptable electrically conductive coins. See, for example, U.S. Pat. No. 3,870,137, assigned to the assignee of the present application.
In some of the tests by apparatus as disclosed in the prior art, the coin under test is introduced through a coin entry and travels along a coin passageway past an inductor or inductors located along one side of the coin passageway or in some cases on opposing sides of the coin passageway. The inductor is part of an inductor-capacitor (LC) oscillator circuit which oscillates at an idling frequency in the absence of a coin. When a coin is present alongside the inductor, the frequency of the oscillator circuit containing the inductor shifts. The degree of interaction between the coin and the electromagnetic field of the inductor forms the basis for coin identification.
Given the natural wear which occurs during a coin's circulation and the consequential range of coin interaction for a given coin denomination, a practical coin identification apparatus must accept coins which fall within certain tolerance ranges. Accuracy of measurement of the interaction of a coin and an electromagnetic field is important and particularly so for discriminating between valid coins just within the tolerance range and invalid coins just outside the tolerance range.